Nodal nitrogen atom

The derivatives of a new condensed triazabicyclo-[5.3.0]dec-4-ene system with a nodal nitrogen atom (7-trifluoromethyl-5-phenyl-l,4,8-triazabicyclo[5.3.0]dec-4-ene 72) are obtained in reactions of aminoenones 71 with diethylenetriamine (room temperature, no solvent, 4-7 days) (yield 64-74% for the trifluoromethyl substituent and 23-33% for tetrafluoroethyl) (99IZV1410) (Scheme 76). 

It was shown (00IZV1432) that the reactions of aminoenones with diethylene-triamine lead to a condensed system, triazabicyclo[5.3.0]dec-4-ene, with a nodal nitrogen atom (Scheme 81). 

When the nitrogen atom and its three substituents lie in the same plane (e.g. in I and in II), the singly occupied molecular orbital (SOMO) is a pure n orbital, and the nitrogen atom lies in its nodal plane. As a consequence, according to (1) the nominally unpaired orbital cannot give any direct contribution to hfs (delocalization term). However, the presence of an electron with unpaired spin (say a) in the n system polarizes the electrons of the a system, d causes a slight excess of a electrons at the nitrogen nucleus in a planar / NO moiety only this spin polarization (indirect) effect contributes to A. ... 

It is notable that pyridine is activated relative to benzene and quinoline is activated relative to naphthalene, but that the reactivities of anthracene, acridine, and phenazine decrease in that order. A small activation of pyridine and quinoline is reasonable on the basis of quantum-mechanical predictions of atom localization encrgies, " whereas the unexpected decrease in reactivity from anthracene to phenazine can be best interpreted on the basis of a model for the transition state of methylation suggested by Szwarc and Binks." The coulombic repulsion between the ir-electrons of the aromatic nucleus and the p-electron of the radical should be smaller if the radical approaches the aromatic system along the nodal plane rather than perpendicular to it. This approach to a nitrogen center would be very unfavorable, however, since the lone pair of electrons of the nitrogen lies in the nodal plane and since the methyl radical is... 

We encounter a different type of bond in a nitrogen molecule, N2. There is a single electron in each of the three 2p-orbitals on each atom (33). However, when we try to pair them and form three bonds, only one of the three orbitals on each atom can overlap end to end to form a (T-bond (Fig. 3.10). Two of the 2/7-orbitals on each atom (2px and 2py) are perpendicular to the internuclear axis, and each one contains an unpaired electron (Fig. 3.11, top). When the electrons in one of these p-orbitals on each N atom pair, the orbitals can overlap only in a side-by-side arrangement. This overlap results in a TT-bond, a bond in which the two electrons lie in two lobes, one on each side of the internuclear axis (Fig. 3.11, bottom). More formally, a 7T-bond has a single nodal plane containing the internuclear axis. Although a TT-bond has electron density on each side of the internuclear axis, it is only one bond, with the electron cloud in the form of two lobes, just as a p-orbital is one orbital with two lobes. In a molecule with two Tr-bonds, such as N2, the... 

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